The present invention relates generally to the art of plasma cutting. More particularly, this invention relates to a method and apparatus for controlling the pilot arc current in a plasma cutting torch.
Many plasma cutting torches have a pilot circuit for initiating a plasma cutting arc which operates upon a workpiece. Pilot circuits require special circuitry and mechanical design to produce a pilot arc which can be transferred to the workpiece to form the cutting arc. A commonly used example of a special pilot circuit is a high voltage, high frequency (HVHF) circuit that starts the pilot arc by breaking down the gap between the torch electrode and anode with a high voltage pulse. The HVHF type pilot circuit, however, has the disadvantage of creating significant amounts of electromagnetic radiation that could interfere with or damage other electronics.
Another common method for starting the pilot arc, called contact starting, uses a specially designed torch and nozzle. An example of a contact start torch is described in U.S. Pat. No. 4,791,268, to N. Sanders, et al., and U.S. Pat. No. 4,902,871, to N. Sanders, et al., incorporated herein by reference. When an operator triggers the contact start torch, the pilot circuit generates the pilot arc between the electrode and the torch nozzle. An ionizable gas is directed through the arc, creating a plasma jet. The plasma torch then is brought near the workpiece to initiate the cutting arc. The plasma jet ionizes the gap between the torch and the workpiece, and the arc transfers, extending from the torch electrode to the workpiece.
In both HVHF and contact starting plasma cutting torches, the current flow in the pilot circuit can be either maintained or interrupted once the pilot arc is transferred to the workpiece. However, the continuous pilot current mode and the interrupted current mode each have disadvantages as well as advantages. For example, if the pilot circuit is kept active in a HVHF type torch, undesirable electromagnetic radiation is continuously generated. In a contact starting torch, maintaining pilot current flow results in excessive wear and tear on the torch nozzle. However, the continuous pilot current mode enables the plasma torch to generate a pilot arc immediately upon loss of the cutting arc, advantageously allowing the operator efficiently to cut broken or expanded metal. On the other hand, if the pilot current is interrupted, electromagnetic radiation from a HVHF torch is minimized and the consumable life of a contact start torch nozzle increases, but at the expense of losing the ability to revert quickly back to a pilot arc and cutting arc. An outage of the cutting arc creates a nuisance when working upon a piece having any type of openings.
One prior art plasma arc cutting power supply uses a mechanical relay in series with the pilot arc circuit. The relay is opened when the arc is cutting, and is closed when the process is being initiated. This mechanical relay includes a snubber circuit in parallel with the relay. The snubber circuit includes a capacitor in parallel with a resistor. The snubber circuit also includes a diode in parallel with another resistor. The resistor/diode combination is in series with the resistor/capacitor combination. The snubber resistors have a combined resistance of approximately 3400 ohms. The pilot arc circuit includes a resistor having a resistance of approximately 6 ohms in series with the relay/snubber circuit. Thus, a small pilot current may flow even when the relay is opened. However, the relatively high snubber resistance limits the current to a less than useful (for minimize arc outages) magnitude. The continuous pilot current will be on the order of 70 milliamps for a pilot current of 15 amps. Also, the mechanical relay cannot be closed fast enough to prevent or minimize arc outages. Indeed, this prior art plasma cutter requires HF to start and restart the arc.
Thus, there is a need for a plasma cutting torch that can prevent or minimize an outage of the cutting arc, that has an increased consumable life, and that does not produce significant amounts of electromagnetic radiation.